The present invention relates to a method and apparatus for filling the pores of hollow solid bodies such as carbon or graphite electrodes (hereinafter called carbon electrodes), and more particularly to improvements in a method and apparatus which can be resorted to for the making of premium grade carbon electrodes which are impregnated with pitch, tar and/or other filler materials (hereinafter referred to as pitch).
The utilization of carbon electrodes for the purpose of melting steel in an electric furnace is gaining increasing popularity. One of the reasons for such increasing popularity is that a furnace utilizing carbon electrodes can be used for the processing of low-grade scrap. As a rule, electrodes of the type used in electric furnaces have a diameter of 300-800 mm and an axial length of 1500-3000 mm.
In accordance with a heretofore known procedure, carbon electrodes are produced by introducing effective amounts of ground coke and pitch into a suitable mold and by baking the contents of the mold at a temperature of approximately 1000.degree. C. in the absence of oxygen. The vapors which develop during baking entail the formation of pores in the resulting electrode. Such porosity, which can amount to a substantial percentage of the volume of the electrode (under certain circumstances up to 20 percent by volume), is undesirable for a number of reasons. As a rule, the manufacturer attempts to produce high-density electrodes because the current-carrying capacity of such electrodes is much more satisfactory than that of highly porous electrodes. Furthermore, an electrode whose density is high is more resistant to thermal shocks, its homogeneousness is better than that of high-porosity electrodes, and the electrode exhibits a much higher degree of elasticity. This is the reason that the makers of carbon electrodes subject such commodities to a secondary treatment which involves impregnation with pitch in autoclaves.
Heretofore known techniques involving pitch-impregnation of carbon electrodes include the insertion of carbon electrodes into a dryer and heating for a period of 2-6 hours to a temperature of approximately 200.degree. C. with gas or oil burners. In the next step, the electrodes are transferred from the dryer into an autoclave by resorting to a crane (such as an overhead service crane), and the pressure in the autoclave is reduced below atmospheric pressure. This ensures expulsion or evacuation of gases from the pores of the electrodes. The next-following step involves the admission of pitch in such quantities that the electrode blanks are fully immersed in the filler. The pressure in the autoclave is thereupon raised by a pump or the like to 6-20 bars so that the pitch is caused to penetrate into the blanks and to fill the pores. The elevated pressure is maintained for a period of 1-2 hours. The surplus of admitted pitch is pumped out of the autoclave, and the electrodes are removed from the autoclave and inserted into cooling basins.
The just outlined conventional pitch-impregnating procedure exhibits a number of serious drawbacks. Thus, the malodorous fumes which develop during treatment of blanks contaminate the surrounding area, and the contaminants which are contained in such fumes are permitted to escape into the air or must be subjected to expensive secondary thermal combustion which consumes substantial amounts of energy. Alternatively, contaminated air must be caused to pass through complex and expensive electrofilters. The procedure is time-consuming and expensive on the additional ground that it involves a substantial amount of manual labor. Still further, the heating of carbon blocks is not uniform, and opening of the autoclave subsequent to evacuation of air and for the purpose of admitting pitch invariably results in penetration of substantial quantities of air into the pores of heated and vacuum-treated blanks. This prevents complete impregnation of the blanks so that the electrodes are of inferior quality or, at the very least, the quality of each and every portion of a finished electrode is not the same. The autoclave which is used for the practice of the aforedescribed process is bulky and the ratio of its capacity to the combined volume of blanks therein is highly unsatisfactory. This, too, contributes to higher cost of heretofore known premium grade electrodes.